Keeping windows and other glass surfaces clean is a relatively expensive, time-consuming process. While cleaning any individual window is not terribly troublesome, keeping a larger number of windows clean can be a significant burden. For example, with modern glass office towers, it takes significant time and expense to have window washers regularly clean the exterior surfaces of the windows.
Windows and other glass surfaces can become “dirty” or “soiled” in a variety of ways. Two of the primary manners in which windows can collect dirt involve the action of water on the glass surface. First, the water itself can deposit or collect dirt, minerals or the like onto the surface of the glass. Obviously, dirty water landing on the glass will leave the entrained or dissolved dirt on the glass upon drying. Even if relatively clean water lands on the exterior surface of a window, each water droplet sitting on the window will tend to collect dust and other airborne particles as it dries. These particles and any other chemicals that become dissolved in the water will become more concentrated over time, leaving a characteristic spot or drying ring on the glass surface.
The second way in which water tends to give a window or other glass surface a soiled or less attractive appearance is tied to an attack on the glass surface itself. As a droplet of even relatively clean water sits on a glass surface, it will begin to leach alkaline components from the glass. For a typical soda lime glass, the soda and lime will be leached out of the glass, increasing the pH of the droplet. As the pH increases, the attack on the glass surface will become more aggressive. As a result, the glass that underlies a drying water droplet will become a little bit rougher by the time the water droplet completely dries. In addition, the alkaline components that were leached out of the glass will be redeposited on the glass surface as a drying ring. This dried alkaline material not only detracts from the appearance of the glass; it will also tend to go back into solution when the glass surface is wetted again, rapidly increasing the pH of the next water droplet to coalesce on the glass surface.
In storing and shipping plate glass, the presence of water on the surfaces between adjacent glass sheets is a chronic problem. One can take steps to shield the glass from direct contact with water. However, if the glass is stored in a humid environment, water can condense on the glass surface from the atmosphere.
This becomes more problematic when larger stacks of glass are collected. Large stacks of glass have a fairly large thermal mass and will take a long time to warm up. As a consequence, they will often be cooler than the ambient air when ambient temperature increases (e.g., in the morning), causing moisture in the air to condense on the surface of the glass. Due to limited air circulation, any moisture which does condense between the sheets of glass will take quite a while to dry. This gives the condensed moisture a chance to leach the alkaline components out of the glass and adversely affect the glass surface. The rate of attack can be slowed down somewhat by applying an acid to the surface of the glass. This is commonly done by including a mild acid, e.g., adipic acid, in the separating agent used to keep glass sheets from sticking to and scratching one another.
A number of attempts have been made to enable a glass sheet to keep a clean appearance longer. One avenue of current investigation is a “self-cleaning” surface for glass and other ceramics. Research in this area is founded on the ability of certain metal oxides to absorb ultraviolet light and photocatalytically break down biological materials such as oil, plant matter, fats and greases, etc. The most powerful of these photocatalytic metal oxides appears to be titanium dioxide, though other metal oxides which appear to have this photocatalytic effect include oxides of iron, silver, copper, tungsten, aluminum, zinc, strontium, palladium, gold, platinum, nickel and cobalt.
While such photocatalytic coatings may have some benefit in removing materials of biological origin, their direct impact on other materials is unclear and appears to vary with exposure to ultraviolet light. As a consequence, the above-noted problems associated with water on the surface of such coated glasses would not be directly addressed by such photocatalytic coatings.
A number of attempts have been made to minimize the effect of water on glass surfaces by causing the water to bead into small droplets. For example, U.S. Pat. No. 5,424,130 (Nakanishi, et al., the teachings of which are incorporated herein by reference) suggests coating a glass surface with a silica-based coating that incorporates fluoroalkyl groups. The reference teaches applying a silicone alkoxide paint onto the surface of the glass, drying the paint and then burning the dried paint in air. Nakanishi, et al. stress the importance of substituting part of the non-metallic atoms, i.e., oxygen in a layer of SiO2, with a fluoroalkyl group. Up to 1.5% of the oxygen atoms should be so substituted. Nakanishi, et al. state that if less than 0.1% of the oxygen atoms are substituted with a fluoroalkyl group, the glass won't repel water properly because the contact angle of water on the glass surface will be less than 80°.
Such “water repellent” coatings do tend to cause water on the surface of the glass to bead up. If the coating is applied to an automobile windshield or the like where a constant flow of high velocity air is blowing over the surface, this water beading effect can help remove water from the glass surface by allowing the droplets to blow off the surface. However, in more quiescent applications, these droplets will tend to sit on the surface of the glass and slowly evaporate. As a consequence, this supposed “water repellent” coating will not solve the water-related staining problems noted above. To the contrary, by causing the water to bead up more readily, it may actually exacerbate the problem.
Other silica coatings have been applied to the surface of glass in various fashions. For example, U.S. Pat. No. 5,394,269 (Takamatsu, et al.) proposes a “minutely rough” silica layer on the surface of glass to reduce reflection. This roughened surface is achieved by treating the surface with a supersaturated silica solution in hydrosilicofluoric acid to apply a porous layer of silica on the glass sheet. By using a multi-component sol gel solution, they claim to achieve a surface which has small pits interspersed with small “islet-like land regions” which are said to range from about 50 to 200 nm in size. While this roughened surface may help reduce reflection at the air/glass interface, it appears unlikely to reduce the water-related staining problems discussed above. If anything, the porous nature of this coating appears more likely to retain water on the surface of the glass. In so doing, it seems probable that the problems associated with the long-term residence of water on the glass surface would be increased.
Most low-emissivity glass articles have an infrared-reflective coating on a protected interior surface of the structure rather than on an exposed exterior surface. For example, in a common automobile windshield having an outer pane of glass laminated to an inner pane of glass with a tear-resistant plastic layer, an infrared reflective coating is commonly applied to one of the glass surfaces immediately adjacent the plastic layer. This helps reduce the transmission of energy as infrared radiation through the windshield, helping maintain a comfortable temperature in the cabin of the vehicle.
However, such an internal infrared reflective coating does not limit emissivity of the outer pane of glass. During the night, for example, the exterior pane of the windshield will lose heat energy to the ambient atmosphere through both convection and infrared radiation to the ambient atmosphere. As a result, the exterior pane of the windshield can cool fairly quickly. When the ambient temperature starts to rise, this cool exterior pane may precipitate moisture from the ambient air, either in liquid form as dew or in frozen form as frost, if its temperature is at or below the “dew point” of the ambient atmosphere when the ambient temperature starts to rise. Providing a low emissivity infrared reflective layer on the outer surface of the glass would reduce heat loss from the outer pane of glass to the ambient atmosphere. While heat would still be lost by convection, limiting heat loss as infrared radiation may keep the glass sufficiently warm to avoid having the glass cooler than the “dew point” and thereby limit or even prevent the precipitation of dew or frost on the surface.
Most conventional sputtered infrared-reflective films are inadequately durable to be carried on an external glass surface. Such films may withstand short-term exposure to the elements during transportation and storage prior to incorporation in an insulating glass (IG) assembly or an automobile windshield. However, they are insufficiently durable to weather indefinite exposure to the elements and usually are assembled in IG assemblies or windshields where they are shielded from the ambient atmosphere by another pane of glass.
Pyrolytic coatings are deposited on the glass surface using relatively high-temperature chemical vapor deposition (CVD) processes, most commonly by contacting a surface of the cooling glass ribbon in the annealing lehr or in the tin bath of a float glass manufacturing line. Such pyrolytically applied coatings tend to be harder, exhibit a different surface morphology, and are able to better withstand exposure to the elements than sputtered coatings of the same composition and thickness. Hence, pyrolytic coatings are better candidates than sputtered coatings for application of a low emissivity coating on the external surface of a window or other glass article.
Unfortunately, pyrolytic coatings have other drawbacks that have limited their widespread commercial adoption for such purposes. For example, one product having a pyrolytically applied tin oxide low emissivity coating is commercially available under the trade name Energy Advantage from Libbey Owens Ford of Toledo, Ohio, USA. This coating has been considered for automotive windshield applications. It apparently tends to adversely affect bonding of the glass to the tear-resistant plastic sheet in such windshield laminates, requiring that it be used as either the external surface (i.e., facing the ambient environment) or the internal surface (i.e., facing the cabin of the vehicle) of the windshield. Applied to the external surface, the pyrolytically applied coating does not appear to be sufficiently durable to withstand the rigors of many years of chemical exposure to the elements and the physical abrasion such a surface must endure. In addition, it has been observed that this coating is notably more difficult to clean when it becomes dirty and tends to become dirty more readily than standard, untreated glass. As a result, it is not deemed an optimal choice for the external surface of an automobile windshield and it has achieved limited success in the marketplace for this application.